1. Field of the Invention
The present invention relates to a process for preparing a metallized polyimide film. More specifically, it relates to a process for preparing a metallized polyimide film comprising an aromatic polyimide layer and a metal plated layer bonded integrally with a high bonding strength or high adhesion through a metal vapor deposited layer, without the use of an adhesive, and the laminated film obtained therefrom. The metallized polyimide films thus prepared can be practically utilized for various purposes, such as a magnetic tape, TAB, and flexible printed circuit substrate.
2. Description of the Related Art
In the prior art, a metallizing film obtained by a conventional metallizing technique for forming a metal layer on an organic film has been generally used for, for example, gold and silver coatings providing a decorative metallic luster and packaging material utilizing a gas barrier characteristic.
Recently, the usage of metallized films has expanded, by utilizing the electrical characteristics possessed by a metal vapor deposited film, for example, when used as a base material of electronic units such as printed circuit substrate.
Known metallized films as mentioned above are generally prepared by a vapor deposition of various metals onto organic films by, for example, the vapor deposition method, or the sputtering method, or the ion plating method, (for example, see Japanese Unexamined Patent Publication (Kokai) No. 54-141391).
Further, a method of preparing an aromatic polyimide film having a high heat resistance from an aromatic polymer composition comprising an aromatic polyamic acid obtained from an aromatic tetracarboxylic acid component such as biphenyltetracarboxylic acids and an aromatic diamine component, such as phenylenediamine, dissolved in an organic polar solvent by the solution casting method, is known from, for example, Japanese Patent Publication (Kokoku) No. 60-42817.
Nevertheless, although the biphenyltetracarboxylic acid type polyimide film prepared according to the above-mentioned known method has a high heat resistance, mechanical strength and dimensional stability, the polyimide film has a poor bonding strength to a metal vapor deposited layer or a metal plated layer, and therefore, the metallized polyimide film prepared by a metal vapor deposition and a metal plating directly onto the above-mentioned polyimide film is not practical from the viewpoint of the bonding strength between the polyimide layer and the metal layer, and accordingly, these metallized polyimide films are not practically used as materials for electronic parts (e.g, printed circuit substrate) requiring a high reliability.
To improve the adhesion strength between the aromatic polyimide film and the metal layer, as described above, attempts have been made to roughen the surface of the film, or to introduce reactive functional groups by an electrophysical surface treatment of the surface of an aromatic polyimide film by a corona treatment or plasma treatment, or by a chemical surface treatment.
Nevertheless, even when using the known means as mentioned above, it is extremely difficult to sufficiently enhance the adhesion between an aromatic polyimide film and a metal layer, such as metal vapor deposited layer, to a practical extent.